During mechanical ventilation the pressure at the external end of the endotracheal tube (ET tube) does not accurately reflect the pressure in the patient's trachea because the ET tube possesses a finite resistance which dictates the presence of a gradient between the two ends of the tube in the presence of flow. During the inhalation phase the external pressure overestimates the pressure in the patient's trachea, the latter being the pressure actually applied to the patient to assist his breathing. In extreme cases, external pressure may be positive, implying that patient is being assisted, while tracheal pressure is negative, whereby the patient's breathing is actually being hindered, rather than assisted, on account of the resistance of the ET tube. Furthermore, because the external pressure during the inhalation phase is routinely used to make inferences about the mechanical properties of the patient's respiratory system, and hence disease progress, and since the external pressure during the inhalation phase includes a component used to overcome ET tube resistance, changes in ET tube resistance, as a result of secretions or kinking, may erroneously lead to the impression that patient's resistance has changed when in fact it has not.
Because during expiration flow is from patient to breathing circuit, external airway pressure (i.e. at the external end of the ET tube) underestimates the pressure in patient's trachea. When ET tube resistance is high, this difference can be significant and the patient may in fact be exhaling against a substantial positive pressure. This would not be detected when only external pressure is being monitored even though it may sufficiently impede expiration as to prevent the respiratory system from reaching its relaxation volume before the onset of the next inspiration (auto-PEEP). It is possible to estimate the pressure difference between the external and internal ends of the ET tube from the magnitude and direction of flow and the known pressure-flow characteristics of the ET tube used in the patient. This difference then can be added or subtracted, depending on direction of flow, from the externally measured pressure in order to obtain an estimate of tracheal pressure to be used for monitoring, estimation of patient's respiratory mechanics or for control of the ventilator. Such an approach has three important drawbacks:
1. It entails the use of flow measuring devices during both inspiration and expiration. Although monitoring inspiratory flow is not problematic and is available on many commercial ventilators, flow meters placed on the common tubing (i.e. between Y connector and ET tube) or the exhalation tubing are subject to water condensation and plugging by secretions which alter their calibrations.
2. Because the resistance of the ET tubes is not linear (i.e. resistance is flow dependent) and varies from size to size, complex electronic mechanisms are required to compute the estimated pressure difference along the ET tube and to subtract or add the difference to the measured external pressure.
3. Estimates of the pressure gradient along the ET tube must be carried out using the pressure-flow relation of the specific tube used. This relation is obtained from standard, clean tubes tested outside the patient. The pressure-flow relation so determined need not reflect the actual pressure-flow relation of the tube while in use. The resistance of the tube is situ is higher by a highly variable amount due to secretions and kinking (Marini, Amer. Rev. Resp. Dis. 140:10-16, 1989).
These problems can all be avoided by measuring the pressure directly at the tracheal end of the tube. At present, this can be done through the construction of home-made catheters and inserting them through improvised access sites in the external tubing. These catheters have to be individually constructed and sterilized and, because the catheter and its tip lie free in the lumen of the tube or in the trachea, they must be removed when suction is to be carried out. In addition, the tip of the catheter is subject to malfunction due to the accumulation of secretions. The free tip of the catheter is also liable to swing and flutter under the influence of the air currents, resulting in pressure artifacts.
As noted above, auto-PEEP is a recently-recognized complication in ventilated patients. With auto-PEEP the respiratory system fails to return to its relaxation volume prior to the onset of the next inhalation phase. Auto-PEEP may have serious consequences with respect to comfort, the respiratory muscles, and the circulation. In some patients, auto-PEEP develops because of an expiratory flow-limitation in the patient's own airway, which cannot be corrected by external devices. Most of the time, however, failure of proper emptying during exhalation is the result of high resistance of the tubing (including the endotracheal tube) and exhalation valve.
To avoid apparatus induced auto-PEEP, ideally airway pressure during exhalation should be controlled at zero (or at a constant positive pressure in the event external PEEP is desired) regardless of the rate of expiratory flow. There are currently no satisfactory means to control airway pressure in this fashion.